1. Field of the Invention
The present invention relates generally to the field of electroactive polymer devices, and, in particular, to a dual conducting polymer charge storage devices/supercapacitors fabricated from poly(3,4-propylenedioxythiophene) and poly(3,4-ethylenedioxythiophene) which operate as electrode couples.
2. Description of the Prior Art
Electroactive polymer devices, in which the polymers store charge and are switched between redox states, have been the object of intense research over the past several years. As these polymers have the possibility of being switched between their neutral form, a p type doped oxidized form, and an n-type doped reduced form, a variety of electrode configurations are possible and highly desirable. This has been illustrated by the use of electroactive polymers in supercapacitors, rechargeable storage batteries, and electrochromic devices.
As a family of polymers, the poly(3,4-alkylenedioxythiophenes) (PXDOTs) have very useful redox switching properties due to their electron-rich character which yields very low switching potentials. The parent polymer of this family, poly(3,4-ethylenedioxythiophene) (PEDOT), has now been developed to the point of commercialization and is used as a stable conducting material in photographic film, tantalum capacitors, and feed through holes in printed circuit boards. In addition, poly(3)3,4-alkylenedioxythiophenes) switch rapidly and efficiently between their neutral and p-doped forms with a minimum of side reactions and long switching lifetimes. Accordingly, poly(3)3,4-alkylenedioxythiophenes) are being heavily investigated for a number of redox devices including electrochromic applications.
A key component in many electrochromic and other redox switching devices is the formulation of solvent-swollen polymer-supported electrolytes. These electrolytes generally consist of a high-boiling plasticizer, a high molecular weight polymer such as poly(methylmethacrylate) (PMMA), and a lithium salt, such as lithium bis(trifluoromethylsulfonyl)imide (Li-BTI). Although this formulation works well, the speed of electroactive switching device is often limited by the conductivity of the electrolyte formulation and the ability of the ions to move into and out of the electroactive polymer layers.
Since an increase in switching speed of these switching devices is highly desirable for many applications, new electrolyte formulations are needed. One such electrolyte formulation to be considered are the molten salts. Electrolytes using the 1-ethyl-3-methyl-1-H-imidazolium (EMI+) cation have shown promise as high speed switching devices. Because of its organic nature, the fact that the 1-ethyl-3-ethyl-1-H-imidazolium (EMI+) cation is less solvated than Li+, and the fact that the cation exhibits a relatively large electrochemical window, makes the cation an excellent candidate for use in gel electrolytes. Furthermore, EMI-BTI is stable up to 300° C., and has an electrochemical window of 4.3 Volts, both of which are highly desirable properties in electrolytes.
It has been suggested that ion-ion interactions in electrolytes provide the following results: (1) Na+ is in a highly complexing environment with AICI4−, while EMI+ is not; (2) salvation for Na+ is higher than EMI+ (Na+ even distorts the AICI4− anion to some extent); (3) EMI+ interacts only weakly with the PF6− anion; (4) dimethylpropylimidizolium cation complexes (via H-bonding) with the Cl− anion, but not with the larger AICI4− anion; (5) Li+ has a fairly high salvation energy, even approaching that of water; and (6) oxidative intercalation of the organic cation into graphite occurs at a potential that is negative of that predicted for Li+ intercalation. This latter fact might result from lower salvation energy of the organic cation and/or a more stable organic-graphite versus Li+— graphite complex.
Another possible explanation is that since Li+ is a small polarizing cation where as EMI+ is larger and less polarizing, weaker columbic interactions in the EMI+-based electrolytes are present leading to a higher mobility of EMI+. It has been reported that Li+ and other cations have a very strong complexing ability, as evidenced by density, molar volummes and thermal expansion coefficients data.
Not all the above are not direct measurements of salvation energy and columbic interaction differences. However, they do provide compelling evidence that the cations in room temperature liquid electrolytes are either not as solvated as alkali metal cations, or the columbic interactions in EMI+-based electrolytes are weaker. Both of these theories suggest that molten salt cations would tend to have higher mobilities. Accordingly, this suggests that devices constructed using room temperature ionic liquids as the supporting electrolyte would tend to switch more rapidly than devices with lithium as the supporting electrolyte.